here is a schematic for a CS-80 style VCO. It is only a LTSPICE simulation until now. A guy asked for help on a german forum ( http://www.sequencer.de/synthesizer/viewtopic.php?f=13&t=78218 ) and I found it interesting to help him, because Yamaha used a closed loop VCO in their Chip. Although closed loop VCOs typically have better linearity than our "bread & butter discharge cap. with switch"-VCOs they are not popular in music electronics, because the rising ramp has a slope with fixed rise time and the amplitude is not constant over CV-range.
Rise time at low freq. does not play a role, but for frequencies in the kHz range , waveform changes towards an asymmetrical triangle. I guess that is the reason why Yamaha added a waveform shaper that simply masks the rising slope with a constant amplitude "punch" from the closed loop one-shot.
the short (30us ?) punch does not add audible harmonic content and, I guess again, is "grinded" away in the (bandwidth limited) signal path.
Please comment

Looks deceptively simple...What would be required to make this a 'practical circuit' in terms of compensation for temperature or other variations which might alter tuning/performance?

elektrouwe wrote:

here is a schematic for a CS-80 style VCO. It is only a LTSPICE simulation until now. A guy asked for help on a german forum ( http://www.sequencer.de/synthesizer/viewtopic.php?f=13&t=78218 ) and I found it interesting to help him, because Yamaha used a closed loop VCO in their Chip. Although closed loop VCOs typically have better linearity than our "bread & butter discharge cap. with switch"-VCOs they are not popular in music electronics, because the rising ramp has a slope with fixed rise time and the amplitude is not constant over CV-range.
Rise time at low freq. does not play a role, but for frequencies in the kHz range , waveform changes towards an asymmetrical triangle. I guess that is the reason why Yamaha added a waveform shaper that simply masks the rising slope with a constant amplitude "punch" from the closed loop one-shot.
the short (30us ?) punch does not add audible harmonic content and, I guess again, is "grinded" away in the (bandwidth limited) signal path.
Please comment

[quote="Yipdeceiver"What would I have to change to run this on +/-12v?[/quote]
It will run @+-12V without problems, but a V/Hz slope change (mostly) from Iref = -V/R9 will happen.
R9 can be changed to a 12/15 lower value to get (almost) the same V/Hz slope as with +-15V.
It might be a good idea anyway to add a trimpot in series to R9 to compensate for other component tolerances.

itist,
Would you care to share more info/details about the sync-isse you had and solved by using a buffer?
Never seen a solution like that before, only seen it solved by using precision voltage reference's and such. This makes me curious!

In my first Impression Video I connected all outputs on the Diode D1 via a summing resistor Network
to an Opamp and then to the soundcard.

My assumption is that the outputs interfere with the trigger circuit (current return path??) and with the unity gain buffer this path is cut off.
Maybe it was only a kind of capacitive coupling. But with the buffer the sync issue has gone.

The circuit ist powered with a standard linear Voltage power supply but the control voltage is created with an LT1236.

For a linear VCO you need, especially in the lower key range, a precise Voltage. The lowest C is 125mV and the next C is 250mV
so each halftone is appr. 10 mV
I will do further testing but in my first tests the VCO seems to be linear over the Voltage Range from 125mV to 4mV.

itist,
Would you care to share more info/details about the sync-isse you had and solved by using a buffer?
Never seen a solution like that before, only seen it solved by using precision voltage reference's and such. This makes me curious!

i will try to reproduce this issue and will do Audio and Scope Recording.
Maybe its synced because of the Scope probes?
It's also necessary to check the second board. On this board one VCO sounds muffled. THe other three sounds crisp.

itist,
Thanks for the info, always interesting to hear other peoples experiences!

Your thoughts makes sense, could very well be like you describe.

I have similar results with some other VCO's i have, but only been planning on solving it by precision voltage reference instead. Gonna give this a go too as there is no buffer implemented atm. (sorry to go off topic, getting carried away with interest)

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